As battery technology changes, for example, from a nickel metal hydride chemistry to a lithium ion chemistry, the operating range of the batteries increases. Such batteries may be used for high voltage applications, such as in hybrid electric or electric vehicles. In vehicle applications, the high voltage battery may provide power not only to high voltage loads, but to low voltage loads after the voltage is reduced, for example, through a DC/DC converter.
In addition to the high voltage battery, a hybrid electric or electric vehicle may also have a low voltage battery, which may be used to power vehicle lighting, engine cooling fans, heated seats, and/or other low voltage loads. If the electrical loads in the vehicle reach a certain level, it may not be possible for the high voltage battery to provide all of the power, and the low voltage battery may need to augment the power supplied by the high voltage battery. This could occur, for example, if the high voltage loads were too great, or because of limitations in a DC/DC converter. In such a case, a minimum level of charge may need to be maintained in the low voltage battery to ensure that there is enough power, for example, to illuminate the head lights. If the low voltage battery is drained too low, it may necessitate automatic shutoff of certain electrical loads to maintain the minimum charge within the low voltage battery. When this “reactive” method of electrical load management is employed, the power may be so low as to necessitate shutting off systems in such a way that it is very apparent to the vehicle occupants. For example, if the charge of the low voltage battery reaches a certain level, it may be necessary to shut off an air conditioning system, which may be very noticeable and undesirable for passenger comfort.
Therefore, a need exists for a system and method of electrical load management for a vehicle that is largely transparent to the vehicle occupants.